1. Technical Field of the Invention
The present invention relates generally to a blood pressure measuring apparatus which applies weak oscillations to a blood vessel in a living body and measures and analyze the oscillations transmitted through the blood vessel to determine the blood pressure continuously, and more particularly to a blood pressure measuring apparatus designed to monitor the oscillations with high sensitivity regardless of electrical noises arising from ac power supply and/or other peripheral devices and an excitation frequency determining method used in the blood pressure measuring apparatus.
2. Background Art
U.S. Pat. No. 5,590,649 issued on Jan. 7, 1997 discloses a blood pressure determining system which performs a continuous non-invasive measurement of blood pressure. This system is designed to estimate the blood pressure by measuring the velocity of propagation of oscillations through a blood vessel based on the fact that the elasticity of the blood vessel changes with a change in blood pressure, and that a change in elasticity of the blood vessel causes the velocity of propagation of oscillations through the blood vessel to change.
FIG. 3 shows the blood pressure determining circuit as discussed above.
The exciter 2 and the oscillation sensor 4 are attached to the patient""s forearm. The cuff 8 is installed around the patient""s upper arm. The oscillator 3 outputs a signal of a frequency f to the exciter 2 to induce perturbations or oscillate the radial artery in the forearm at the frequency f The frequency f is within a range of 100 Hz to 200 Hz. The oscillation sensor 4 detects oscillations transmitted through the radial artery and converts them into an electrical signal. The amplifier 5 amplifies the output from the oscillation sensor 4 and provides it to the A/D converter 6. The A/D converter 6 converts the input into a digital signal and outputs it to the processor 7. The processor 7 performs a phase detection to determine a change in phase, as viewed from the center of an arc distribution, which indicates a change in velocity of propagation of the oscillations corresponding to a change in blood pressure. This change is calibrated by values of systolic and diastolic pressures measured through the cuff 8 and the blood pressure measuring device 9 to determine the blood pressure continuously. The waveform of the change is indicated on the display 10. The dc power supply 11 is connected to an ac power supply (not shown) through the plug 12 and provides a dc constant voltage to each block.
It is important for precise measurement of the blood pressure in the above system to detect weal oscillation transmitted through the blood vessel with high sensitivity. The power spectrum of a digital signal into which an output of the oscillation sensor 4 amplified by the amplifier 5 is converted by the A/D converter 6 when the exciter 2 is at rest is shown in FIG. 4 which represents the spectrum of noises added to the system. A frequency component within a range b is a patient""s pulse component.
FIG. 4 shows that the power of noise rises at each of integral multiples of the frequency fs of the ac power supply. This is caused by the fundamental and harmonics of an output from the ac power supply entering the input of the system, which will compromise the sensitivity of detection of the oscillation extremely.
The frequency fs of an output from the ac power supply to the dc power supply 11 is different between used devices or countries. It is, thus, difficult to specify the frequency fs in advance.
The frequencies fd1 and fd2 in FIG. 4 are also equivalent to those of vibrations or electromagnetic noises produced by equipment used together with a blood pressure measuring system of the above type in, for example, an operating room that are factors of decreasing the sensitivity of detection of the oscillation. It is difficult to specify the frequency of the noises in advance because peripheral devices used along with the blood pressure measuring system are changed depending upon operating environments.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide a blood pressure determining system designed to measure the blood pressure accurately regardless of electric noises arising from an ac power supply and/or other peripheral devices.
According to one aspect of the invention, there is provided a blood pressure determining apparatus which comprises: (a) an exciter applying oscillations to a blood vessel of a subject; (b) an oscillation sensor monitoring the oscillations propagated through the blood vessel to provide an electric signal indicative thereof; (c) a signal processor including a blood pressure determining circuit determining a blood pressure of the subject based on the signal from said oscillation sensor; (d) a signal analyzer analyzing a frequency of the signal from said oscillation sensor to provide a signal indicative thereof; and (e) a frequency controller controlling a frequency of oscillations to be produced by said exciter based on the signal from said signal analyzer.
In the preferred mode of the invention, said signal analyzer analyzes the frequency of the signal which is outputted from said oscillation sensor when said exciter is in an off-state to determine a small noise frequency band in which a power of electric noise is small. The frequency controller sets the frequency of oscillations to be produced by said exciter to any value of the small noise frequency band.
The signal processor may alternatively define a plurality of frequency ranges in the small noise frequency band and performs a text to apply oscillations to the subject at a frequency that is a central one of each of the frequency ranges to select one of the central frequencies which allows a waveform of the blood pressure in the outputs of the oscillation sensor to be detected with the best sensitivity as the frequency of oscillations to be produced by said exciter.
According to another aspect of the invention, there is provided a method of determining a frequency of oscillations to be applied through an exciter in a blood pressure determining apparatus to a blood vessel of a subject to monitor a frequency of the oscillations propagated through the blood vessel to monitor an output of an oscillation sensor for determining a blood pressure of the subject continuously and non-invasibly, which comprises the steps of: (a) monitoring an output of the oscillation sensor when the exciter applies no oscillation to the subject to determine a power spectrum of noise signals contained in the output; and (b) setting the frequency of oscillations to be applied to the blood vessel of the subject through the exciter to a value within a band of the power spectrum in which a power of noise is small. This enables the output of the oscillation sensor to be provided without any electric disturbances.